Liquid carbon dioxide breakouts rather than water probably created the martian gullies discovered last summer in high-resolution images from the Mars Global Surveyor orbiter camera.

Donald S. Musselwhite, Timothy D. Swindle and Jonathan I. Lunine of the University of Arizona Lunar and Planetary Laboratory publish their hypothesis in today's issue of Geophysical Research Letters.

Last June, scientists announced that gullies seen on some martian cliffs and crater walls suggest that liquid water has seeped down the slopes in the geologically recent past.

Researchers found small channels on slopes facing away from midday sunlight, with most channels occurring at high latitudes near Mars' south pole.

The scientists concluded that the relationship between sunlight and latitude may indicate that ice plays a role in protecting the liquid water from evaporation until enough pressure builds for it to be released catastrophically into the surface.

If channels are forming today, liquid water may exist in some regions of Mars barely 500 meters beneath the surface, they suggest.

Now UA researchers propose an alternative explanation involving carbon dioxide erosion. They point to several reasons why CO2 is a better candidate than water in gully formation.

One reason is that most gullies are found in the southern highlands, the oldest and coldest part of the planet, a place where liquid water is least likely to be stable.

"That's high altitude in a region of low geological activity. It is difficult to invoke some hydrothermal action there," Musselwhite said. "The surface is old, but the gullies are new."

Another reason is that the southern hemisphere has more extreme temperature variations throughout the year than does the northern hemisphere, since Mars is closer to the sun during southern summer and farther away during southern winter, Musselwhite said. The gullies are generally on pole-facing slopes where they receive very little or no sunlight for most of the year.

However, Musselwhite said, the most compelling fact is that gullies always start about 100 meters below the top of the cliff. At that depth, the pressure of the rock overhead is just enough for liquid CO2 to be stable, if the temperature is low enough.

"There are many interesting ideas about how liquid water might carve these things. Still, if the process works in these very special locations where at least during wintertime it is extremely cold, why don't we see the gullies in other places? If you have water cutting these gullies, you should see that everywhere, not just at these specific locations. And where is the water coming from? There is not much of it in the martian atmosphere or on the surface," he said.

It's not liquid carbon dioxide flowing in the gullies.

"What's coming out is liquid CO2 that suddenly vaporizes," Musselwhite said. "As it comes out, it expands very quickly, cools, and actually produces CO2 snow. The snow is suspended in CO2 gas that hasn't solidified yet. Together with rock debris, it forms slurry. Geologists call it a 'suspended flow.' Suspended flow acts like a liquid. It doesn't take very much liquid each time to add to gully formation."

There are analogs on Earth to this process.

Martian gullies look almost identical to terrestrial ones found in polar regions and also on cliff walls, where gullies are carved by snow pack. Such channels can also be found on the flanks of Earth volcanoes, carved by a suspended flow of ashes entrained in volcanic gas. And trapped mud and sediment caught in turbidity currents on the ocean floor cut deep channels into the continental shelves, Musselwhite noted.

How do Martian gullies form?

The planet's atmosphere is mostly composed of CO2. Under some atmospheric pressure, CO2 condenses from the atmosphere and into Mars' surface. Mars has been pummeled by impacts, making its surface typically porous, spongy gravel. Gas seeps through the surface and condenses in the pores of rock.

"In wintertime the cliff surface gets so cold that its temperature falls below the freezing point of CO2, which at low pressure goes directly to solid. As the cold wave moves from the surface, the pore space is completely filled in.

"When spring comes, dry ice warms up and expands. Since all the rock pore space is filled, pressure builds until the ice turns to liquid. Liquid CO2 takes up more volume than dry ice, so pressure continues to build."

At the same time, the dry ice dam evaporates and thins as temperature rises. At some point, the barrier becomes too thin and the liquid under pressure bursts out. It breaks through the surface into the atmosphere, where it evaporates quickly due to the sudden drop in pressure.

As carbon dioxide vaporizes rapidly, it also cools and entrains the CO2 snow, creating the suspended flow.

Some researchers claim that the gullies are very young and may currently be forming on Mars. They tie gully locations to oscillations in the martian climate caused by varying tilt of the planet's rotation axis, called obliquity.

When the obliquity is low, Mars' axis is almost straight up and the surface near the poles gets less heating all year around. At high obliquity in winter, more of the surface would be shaded, but in the summer, it would get much more sunlight than usual.

"If this explanation is correct, gullies are forming today around the south pole," Musslewhite said. "The ones that are farther from the poles are then older. You might expect these to form close to the equator in the period of high obliquity, when the axis is more tilted over. Some may be forming now on a yearly basis."

This idea is supported by evidence that some researchers say suggests that gullies are forming today near the south pole but not closer to the equator. Multiple images of the same gullies are needed to prove that, Musselwhite added.